Add doesUsenameExist query

Reviewed-on: #107

src/operations/user.gql

@@ -91,11 +91,14 @@ query GetUserTags {
 mutation followUser($followedUserId: Int!) {
   followUser(followedUserId: $followedUserId) {
     username
+    id
     following {
       id
+      username
     }
     followers {
       id
+      username
     }
   }
 }
@@ -103,17 +106,21 @@ mutation followUser($followedUserId: Int!) {
 mutation unfollowUser($followedUserId: Int!) {
   unfollowUser(followedUserId: $followedUserId) {
     username
+    id
     following {
       id
+      username
     }
     followers {
       id
+      username
     }
   }
 }
 
 query getUserFollowingFollowers {
   getLoggedInUser {
+    id
     following {
       id
       username
@@ -124,3 +131,7 @@ query getUserFollowingFollowers {
     }
   }
 }
+
+query doesUsernameExist($candidateUsername: String!) {
+  doesUsernameExist(candidateUsername: $candidateUsername)
+}